Known two-wheel-drive two-wheeled vehicles (for simplicity referred to as a "2.times.2 vehicle") comprise a frame, a rear wheel having a driven hub that is rotatably mounted on the frame, a front wheel support rotatably mounted on the frame for steering the vehicle and connected to handle bars for steering the front wheel, a front wheel having a hub rotatably mounted on a front wheel support, driving means for driving the vehicle, rear wheel transmission means for transmitting drive from the driving means to the driven hub of the rear wheel, and front wheel transmission means for transmitting drive from the driving means to the front wheel including a first drive member to transmit drive to a first drive element positioned on the frame adjacent the front wheel support, a second drive member mounted on the front wheel support and constructed to transmit drive from a second drive element to a front wheel drive element on the front wheel and drive coupling means to transmit drive from the first to the second drive element. Hereinafter a vehicle of this type is referred to as a "2.times.2 vehicle of the type set forth". 2.times.2 motorcycles are disclosed in an article entitled "The Gripped On Factor" on pages 48 to 53 of a magazine publication "Performance Bikes", February 1990. The article describes the development of such motorcycles by various persons and says that such bikes are more stable and less skittish than the equivalent rear-wheel drive machines in most situations and particularly in slippery conditions. The article also states that taking drive to the front wheel has many problems, such as routing of chains or shafts to the front of the bike, increased weight, drive efficiency, ensuring loads are not fed into the suspension or steering, and driving the wheels so that the frame is not compressed or stretched.
The main problems are transferring drive to the front wheel, because the front wheel and handle-bar assembly as a whole must turn relatively to the frame for steering, and accommodating suspension travel of the front wheel. To accommodate these problems the developers use a constant velocity joint at the steering axis, and two chain drives in series from the output of the constant velocity joint to the front wheel hub. The chain drives in series are carried on a pair of links. While cumbersome this arrangement was chosen over shaft drives because of concern about unwanted torque reaction and over fluid drives because of inefficiency. This arrangement, while perhaps suitable for motorcycles is believed to be heavy, bulky and expensive for bicycles and cannot easily be adapted to existing bicycles or bicycle frames or designs.
A 2.times.2 bicycle of the type set forth is described in a French mountain bike magazine entitled "Velo Vert", of approximately January 1991. The bicycle has a specially constructed front wheel support or fork mounted on a modified head tube, with the drive coupling means passing through the fork and/or head tube assembly. Details of the coupling are not known, but it is surmised that it contains a universal joint or gear set. A disadvantage of this arrangement is that it cannot be retrofitted to an existing bicycle or be easily fitted to existing bicycle designs as it appears to be usable only with a bicycle having a specially constructed head tube and front fork that can accommodate the coupling means as well as support input and output shafts for the chain drives. Another disadvantage is that it apparently cannot accommodate suspension travel of a front wheel that is mounted on a front wheel support that includes shock dampers or springs.
The inventor of the present invention has been developing a 2.times.2 bicycle independently of the developments mentioned previously and has experienced some of the problems mentioned above, particularly the relatively large steering movements of a bicycle compared to those of a motorbike, especially those of so called "mountain bikes" and "moto-cross bikes", and providing a construction that can be fitted to at least some types of existing bicycles and bicycle designs.
Thus, this invention seeks to provide a two-wheel-drive arrangement that is satisfactorily lightweight, inexpensive, efficient, suitable at least for bicycles, and accommodates at least some of the problems and disadvantages of the known designs described above.